The present invention relates to a rotary head type digital-audio reproducing device suited to reproduce digital signals which have been converted into PCM signals and recorded on oblique tracks formed one at a time on a tape like recording medium by means of rotary heads.
There has been proposed a rotary head type digital audio recording/reproducing device which is referred to as an R-DAT (Rotary Head Type Digital-Audio Tape Recorder). In such as recording/reproducing device, the audio signals are recorded on a magnetic tape by a helical scan type rotary head while forming the oblique tracks one at a time. In the next step, the audio signals are recorded and reproduced after the audio signals have been converted into PCM signals.
A format of the tracks on which actual recording is effected in the R-DAT assumes the pattern illustrated in FIG 1(a). Each frequency of MARGIN, PLL, POSTAMBLE is (1/2)f.sub.M (.sub.fM =9.4 MHz). and the frequency of IBG is (1/6)f.sub.M. SUB and PCM are composed of such blocks as illustrated in FIG. 1(b). SYNC is 10 bits (9 bits are fixed). and the rest take a variety of patterns according to their locations and voice signals. The SUB has 8 blocks, and the PCM has 128 blocks. Each numerical value of FIG. 1(a) indicates the number of blocks occupied by the individual regions.
Two regions ATF1 and ATF2 (ATF stands for: Automatic Track Finding are) interposed between the SUB-1 and the PCM and between the PCM and the SUB-2, respectively. These two regions are provided so that tracking control, under which the rotary heads properly scan on the recording tracks at the time of reproduction, can be effected by outputs of the rotary heads, without the use of a special head.
Namely, the ATF regions are regions in which tracking pilot signals are individually recorded. The ATF regions are separate from the recording region for the PCM signals and are provided both at the starting portion of each track (before the PCM region) and at the terminating portion thereof (after the PCM region). When recording the PCM signals on the magnetic tape by use of the rotary heads while forming the oblique tracks thereon, without any guard bands, the tracking pilot signals are recorded after the PCM signals have been time-axis-compressed The pilot signals recorded in the ATF regions are utilized to control the tracking of the rotary heads. Control of the rotary heads is in response to reproduction signals of the pilot signals from two tracks adjacent to respective sides of the track being scanned. The reproduction signals are obtained in the output of each rotary head when reproducing a magnetic tape by scanning the recording track using rotary heads which each have a scanning width larger than the width of the track.
As depicted in FIG. 2, track patterns in the ATF regions are initially determined. Where a drum diameter is 30 mm, a drum winding angle is 90.degree. and the velocity of rotation is 2000 rpm, the illustrated patterns are explained as follows:
The ATFs 1 and 2 disposed at the front and rear portions have signals f.sub.1 having low frequencies which will produce fine azimuth effects. The signals f.sub.1 are defined as the tracking pilot signals. The ATFs 1 and 2 are utilized for detecting a magnitude of the level of cross talk from the two adjacent tracks at the time of reproduction and for obtaining a level difference as a tracking error signal between cross talk components of the two adjacent tracks. The pilot signal f.sub.1 involves a low frequency signal of f.sub.M /72 (=130 KHz).
The ATF-1 and ATF-2 regions also contain SYNC signals for determining the positions in which the pilot signals f.sub.1 are recorded. If cross talk is present, the ON-track (the track being scanned) will be indistinguishable from the two adjacent tracks. Hence, the SYNC signals eligible for selection have frequencies which produce the azimuth effects and have patterns which do not appear in the PCM signals. If the rotary head corresponding to (+) azimuth is A and the rotary head corresponding to (-) azimuth is B, the SYNC signals are arranged to be different from each other in order to distinguish the A rotary head from the B rotary head. The SYNC 1 signals f.sub.2 each having a frequency f.sub.M /18 (=522 KHz) with respect to the A head are recorded in a predetermined location, and similarly the SYNC 2 signals f.sub.3 each having a frequency f.sub. M/12 (=784 KHz) with respect to the B head are recorded in a predetermined location.
No erase head is provided in the R-DAT. When rewriting the signals, an overwrite on the previous record is performed. For this reason, erase signals f.sub.4 having a frequency f.sub.M /6 (=1.56 MHz) are recorded in predetermined positions suitable for erasing the previously recorded pilot signals f.sub.1, SYNC 1 signals f.sub.2 and SYNC 2 signals f.sub.3.
All the positions of the pilot signals of the ATFs differ from each other on the ON track and the adjacent tracks. The level of the pilot signals on the ON-track is different from that of the pilot signals on the adjacent tracks in terms of time. The pilot signals are allocated to sample each of the three levels.
Five blocks are allotted to each of the regions ATF-1 and ATF-2, and the pilot signals f.sub.1 are recorded in two of the five blocks. The SYNC signals f.sub.2, f.sub.3 are recorded by occupying one block or one-half block extending from the center of the recording location of the tracking pilot signals of one of the adjacent tracks. The pilot signal f.sub.1 of the other adjacent track is recorded to locate its center after two blocks from the beginning of the SYNC signal recorded on the ON-track. The SYNC signals of one block are allocated to odd numbered frames, whereas the SYNC signals of one-half block are allocated to even-numbered frames.
As explained above, in the ATF regions, the frequencies of the SYNC signals differ, depending on the A rotary head or the B rotary head, and the recording lengths of the SYNC signals differ, depending on the odd-numbered frames or the even-numbered frames, The four consecutive tracks are distinguishable because all the four tracks are provided with different ATFs. The above described ATF pattern is a 4-track-completion type in which the pattern is repeated for every four tracks.
When reproducing the magnetic tape on which the recording is effected in the format illustrated in FIG. 1(a) by means of the rotary head, RF signals shown in FIG. 3(a) are obtained from the rotary head. If the RF signals are obtained by the reproduction from, for instance, the (A) odd-numbered track frame shown in FIG. 2 the pilot signals f.sub.1 shown in FIG. 3(b) are obtained by use of a band pass filter of 130 KHz.
A section I is associated with the pilot signal on the ON-track. Sections II and III are associated with the cross talk of the pilot signals of the (B) odd-numbered track frame and of the (B) even-numbered track frame. When the rotary head adequately scans on the ON-track, envelope levels of the section II and III, i.e., V.sub.II and V.sub.III shown in FIG. 3(c), are originally equal. If a track-deviation is created, V.sub.II .noteq.V.sub.III. A quantity and direction of deviation of the rotary head in connection with the ON-track can be recognized from the magnitude and the polarity. A capstan servo is operated when a difference between V.sub.II and V.sub.III is detected, and a velocity of the tape is slightly adjusted, thereby causing the rotary head to run on the OM-track.
The requirements for such operations are that the SYNC signals, which have been positioned in predetermined locations, are exactly detected, and the levels of V.sub.II and V.sub.III are sampled. The R-DAT is not, however, provided with an erase head, so the second and third recordings are performed by overwriting. As a result, it is in some cases impossible to generate appropriate error signals by exactly detecting the SYNC signals and sampling V.sub.II and V.sub.III.
Namely, the R DAT permits the recording to be done within plus or minus two blocks from the center of the PCM region, and the recording level of the pilot signal f.sub.1 (=130 KHz) is a little bit lower than those of other signals during recording. The reason for this lower recording level is that as the frequency of the signal is lowered, its recording level to the tape is deepened, and hence it is required to erase the previously recorded pilot signals f.sub.1 by erase signals f.sub.4 when overwriting occurs. Although the levels of the pilot signals f.sub.1 are thus decreased, some of the preceding SYNC signals often remain unerased while the pilot signals f.sub.1 are newly recorded in the location of the previously recorded SYNC signals f.sub.2 or f.sub.3.
More specifically, if the subsequent recording is effected by deviating more in the forward direction than the preceding recording, the SYNC signals of the subsequent recording are invariably prior to the remaining, unerased SYNC signals of the previous recording on the track, which situation is not a problem. However, if the subsequent recording deviates in the backward direction, the remaining, unerased SYNC signals precede those of the subsequent recording. In this case, deviation ranges from one block to two blocks. The SYNC signals f.sub.2 and f.sub.3 of the previous recording partially or wholly remain in the locations of the pilot signals f.sub.1 in the (A) even-numbered frame and (A) odd-numbered frame of the ATF-1 and in the (B) even-numbered frame and the (B) odd-numbered frame of the ATF-2.
When this happens, the level of the frequency component of the pilot signal within the reproduced RF signals is sampled at that time in accordance with the SYNC signals of the previous recording. This pilot signal originally has to assume a level of cross talk of the sampling signals on one adjacent track. However, the frequency component to be sampled is the pilot signal itself on the ON track. The level obtained by sampling achieves a considerably large value. Thereafter, the frequency component of the pilot signal within the reproduced RF signals subsequent to two blocks is sampled. A difference between the thus acquired sample value and a sample value before two blocks is obtained. The capstan servo is controlled on the condition that this level-difference is defined as a quantity of track-deviation. The previously sampled frequency component, however, assumes the level of the ON-track rather than the level of the cross talk of the contiguous track. Consequently, a level which is extraordinarily greater than the quantity of the actual track-deviation is obtained. Such being the case, the capstan servo undergoes a relatively large disturbance which will have undesirable effects on the condition under which the tape is fed.